This invention relates to method for encoding information and for locating, reading, and writing information stored on a disk.
In systems that store data on sectors of a magnetic disk, the stored header that identifies each sector may be encoded to reduce the effect of bit errors that occur in recording. The header typically includes an address that uniquely identifies the sector and may include additional information, such as a bad sector flag and a servo correction number. To locate a particular sector on a specified track of the disk and to read or write data in that sector, the header of each sector along that track must be read and analyzed quickly.
To determine if a sector is the one being sought the sector address on the disk is read and compared with the specified address of the desired sector in which data is to be read or written. If the addresses match, the data will be read from or written to the sector.
If a sector address read from the disk has errors, the sector may be mistakenly identified, causing the disk drive to incorrectly determine, for example, that the head is located over sector 100 when it is actually located over sector 101. As a result, the read or write operation that is to occur at sector 100 will be performed on the wrong sector. If a sector address contains several errors or the address is corrupted due to a medium defect or soft error in the disk, the sector address may be unreadable making the sector unusable.
To avoid misinterpreting the address of a sector, the sector addresses must be written on the disk in such a way that even with a number of errors in the address, it will neither be read as the address of a different sector nor be rendered unidentifiable.